All tank guns in World of Tanks are breech-loading rifles, so no need to discuss smoothbores, muzzle-loaders or any other variants. The four statistics you are most worried about are accuracy, penetration, rate of fire, and damage caused.
Accuracy.
A number of factors determine the accuracy of a gun. Primarily, these are the construction of the gun itself, the design of the cartridge, and the fire control system. Variances or inaccuracies in any of these will result in a decrease in accuracy. No gun as yet built is ‘perfectly’ accurate. The level of inaccuracy inherent in a typical gun is measured in either mils or minutes of angle. The latter is more familiar to civilian rifle shooters, AFV crewmen will be far more familiar with mils. The accuracy figure for a gun in World of Tanks is measured in milsx10.
A mil is effectively the angle described by an item one meter in length when viewed at a distance of 1,000 meters. (or 1 foot at 1,000 feet). Mathematically there are 2000x Pi mils in a circle (about 6,283). For ease of mental arithmetic, NATO militaries round this off to 6,400, though not all countries do so. Modern tank sights are equipped with mil scales, so that if you know the size of a target, you can determine how far away it is. For example, a 2m high tank at 1,000m will appear 2 mils high. At 2,000m, it will appear 1 mil high, and so on. Of course, this is more an academic issue, as range-finding is automatic in World of Tanks.
More importantly, the figure for accuracy will tell you the chances of hitting a target at a given range. As the figure for accuracy is in tens of mils, a gun with an accuracy figure of 0.35 will place its shot within a circle 3.5m in radius at 1,000m. Or, if you wish, a circle 3.5m wide at 500m. This is significant. For example, a King Tiger is only 3.1m high. It is quite possible for a ‘perfectly aimed’ shot with a 0.35 accuracy gun to miss a King Tiger at 500m. It probably won’t, as the dispersion is on a Gaussian curve, but a ‘miss’ would not necessarily mean that you had done anything wrong.
Factors of the gun which affect accuracy are barrel length, stability, temperature distribution and simple production quality control. Generally speaking, a longer barrel will result in a more accurate gun for two reasons. Firstly, the projectile spends more time being affected by the rifling, thus imparting better spin. Secondly, as the projectile spends more time in the barrel, it thus means that it has the propellant effects of gasses acting upon it for a longer time, meaning greater acceleration time and thus a faster projectile. Faster projectiles gain the benefits of a shorter time of flight, meaning that less lead is required to hit moving targets, and there is less time for external factors such as crosswinds to take an effect. They also result in a flatter trajectory, allowing some error in range calculations. In real-world terms, this latter allows for a longer ‘battlesight’ range: Battlesight is a default range which is used when no range information is available: It is a range setting at which, up to a certain distance (about 1.3x battlesight range) a shell is effectively guaranteed never to be too high in its arc of flight to hit a target. Although range calculations are, as mentioned, automatic in World of Tanks, arc of flight can by important especially when leading at targets, as a slow round will fly a higher arc in order to hit the point of ground your sight is aimed at which is in front of, and beyond, your target tank. So why aren’t all cannons as long as possible? Several reasons. Firstly, they just get heavy and droop, bend and flex. Especially large caliber ones. Secondly, you reach a point of diminishing returns, as the longer that the projectile is in the barrel, the more time the friction of the rifling has to impart drag. Further, if it’s really overdone, the propellant gasses stop expanding, and a vacuum is created behind the projectile, further decelerating it.
Then you have the factor of the round fired: Just how it is designed, the aerodynamic efficiency or balance. Big fat rounds like a 152mm HE are going to have pretty major drag issues, reducing its effective range even further than its already short distance caused by the usually short barrels its fired out of. The other extreme is also possible, the APDS round fired by the 17-Pounder was notorious for its inaccuracy.
Fire control systems are probably the least critical for our purposes: Variances in the quality of the optics and the reticles can have an effect, but there’s not much to be done about it as a player in World of Tanks. Spotting range and aiming times are a general mechanic to reflect overall abilities in target acquisition, and target engagement, two entirely separate issues both of which can be affected by a tank's optics. It's possible to have excellent engagement capabilities while having terrible acquisition abilities, as a visit to the gunner's seat of a Panther would show.
Penetration.
There are two methods of traditionally penetrating armour. Good, old-fashioned, brute force, and chemical effects (i.e. explosions).
Most AP rounds work on the brute force principle: A metal ‘arrow’, piercing the skin of the target by sheer inertia (mass by velocity). Shaped charge (HEAT rounds particularly) produce this effect by way of concentrating the force of an explosion into a focused area, then punching through the armour at that point. Regular High Explosive rounds have limited ability to punch through armour, and just blow up, more usually causing structural or component damage, and knocking the crew inside about a fair bit. HESH/HEP doesn't try to penetrate at all, and just relies on transmitted vibration.
The ability to penetrate armour in the game is represented in terms of millimeters of armour at perpendicular impact. This is a variable figure, actual penetration of any particular round fired may be up or down a bit by up to 25%. As an AP round has velocity as an important factor in its penetrating capability, its ability to punch through armour will decrease the further away a target is. This is due to the air resistance acting on the projectile slowing it down over time. Chemical effect rounds such as HE or shaped charge retain their effectiveness over distance.
After it penetrates, the next question is ‘how much damage is caused’? This is where WoT deviates somewhat from real life: After all, real tanks don’t have damage points. However, the general gist of it is right to some extent. A tank may well survive a penetration by a small round better than a large one. In WWII, most AP shells had an explosive content: After impact on the armour, a delayed fuse would set off a small explosive charge which, in theory, would detonate after the round had passed through the armour plate, causing a significant emotional event inside the tank. The larger the caliber of the round, the greater the explosive payload within the projectile, with obvious reprecussions. Later munitions had no explosive content at all, and relied simply on the projectile bringing particles of metal from the armour (a process known as spalling) inside the tank with it, which would then fly about with generally unpleasant consequences. This made them more effective at penetrating armour, but does lead to the problem of over-penetration: If there’s not much armour being penetrated, the round may simply pass all the way through the vehicle and if it doesn’t hit anything vital on the way, just leave two ventilation holes with no other significant effect due to not having brought many armour particles with it on its way through.
Finally, rate of fire. A high rate of fire means that you can generally afford to take snapshots. If you miss, you’ll have another round in a couple of seconds. It also means that if you do hit and cause damage, you can do it again in pretty short order. However, generally speaking, the larger the round, the slower the rate of fire. Anything over 120mm is going to be very slow as 120mm is about the maximum size that a human loader can handle unassisted. A larger caliber gun is liable to give you a very sizeable amount of damage when it hits, but after it’s fired, it’s not going to be a threat to you again for a moderate amount of time. On the other hand, if a target is fleeting, it may well be better to hit him with a big gun once, instead of hoping that the target will hang around long enough for two or three shots with a smaller, faster gun.

Not to argue with The Chieftain but just to get my facts straight, I have read different reasons for the 17-pdr and 77mm issues in the article. It was my understanding that the 17-pdr APDS ammo lost accuracy due to the sabots hitting the muzzle brake and that the smaller propellant charge (thus smaller casing) of the 77mm was to allow using 17-pdr projectiles in turrets too small for the 17-pdr breech. Any clarifications on this would be appreciated.

The other extreme is also possible, the APDS round fired by the 17-Pounder was notorious for its inaccuracy due to the fact that the round was overpowered and thus destabilized in flight. (The fix for this, as used in Comet, was to cut back the gun and propellant a bit, thus making the 77mm gun)

Huh? I thought the issue was clean sabot separation, and the 77mm was made because the 17-pdr wouldn't fit in any cruiser tanks?

I'm sorry to nit-pick here, Chieftain, but Inertia isn't m*v. Momentum (P) is m*v. Inertia is a property of matter.

"Inertia is the resistance of any physical object to any change in its motion. In other words, it is the tendency of objects to keep moving in a straight line at constant linear velocity, or to keep still."

Inertia is described in Newton's first laws of motion. I think what you meant was either "by sheer Momentum" or "by sheer Kinetic Energy"

this topic sounds like good info but this game is only giving certain tanks the ability to shoot on the move at high speed with 100 % kill shots .
this has happened to me a number of time when tanks come around the corner of a rock or building at full speed and make a 100% kill shot on me .
also if I am sitting still and if I aim my gun at a target I wait 2 to 3 seconds for the reticle to start to close then what ever time it takes for the reticle to close and then the shot bounces or some times I get a ghost round or if lucky get a hit but do little damage and the time from starting the aiming to the time the shot is made is 6 or 7 seconds explain that .
these are the problems I have so it makes me wonder why someone can run around a corner or building or rock at full speed and make a 100% kill shot , I have never been able to do it in any of my tanks.

Good post and all but I think most people who can aim are more concerned with getting a -25% RNG roll. Dont matter what you do or how good you are if you get bad rolls you get bad shots. The more WG gets their hands in determining if I will be successful or not in win rate or RNG, the less it makes me want to play if my stats are already predetermined. How many sub 40% games or -25% RNG does it take for someone to get tired of it. Trolls can call me a whiner if ya want dont give a crap about trolls, but all my stats have a positive trend only winrate and accuracy fluxuates and that is out of my control most of the time. I wait 3-4 secs before I fire after reticle is closed and still miss multiple times at 100m.

this topic sounds like good info but this game is only giving certain tanks the ability to shoot on the move at high speed with 100 % kill shots .
this has happened to me a number of time when tanks come around the corner of a rock or building at full speed and make a 100% kill shot on me .
also if I am sitting still and if I aim my gun at a target I wait 2 to 3 seconds for the reticle to start to close then what ever time it takes for the reticle to close and then the shot bounces or some times I get a ghost round or if lucky get a hit but do little damage and the time from starting the aiming to the time the shot is made is 6 or 7 seconds explain that .
these are the problems I have so it makes me wonder why someone can run around a corner or building or rock at full speed and make a 100% kill shot , I have never been able to do it in any of my tanks.

Kombaticus, on Nov 10 2013 - 00:46, said:

Good post and all but I think most people who can aim are more concerned with getting a -25% RNG roll. Dont matter what you do or how good you are if you get bad rolls you get bad shots. The more WG gets their hands in determining if I will be successful or not in win rate or RNG, the less it makes me want to play if my stats are already predetermined. How many sub 40% games or -25% RNG does it take for someone to get tired of it. Trolls can call me a whiner if ya want dont give a crap about trolls, but all my stats have a positive trend only winrate and accuracy fluxuates and that is out of my control most of the time. I wait 3-4 secs before I fire after reticle is closed and still miss multiple times at 100m.

"After impact on the armour, a delayed fuse would set off a small explosive charge which, in theory, would detonate after the round had passed through the armour plate, causing a significant emotional event inside the tank"
LOL
BTW, I really liked this read +1. Stuff like AP rounds passing through armor and causing 2 ventilation holes or fuse inside the AP shell can be confirmed from my other sources I saw/read - I saw youtube video of german tanker that during the war said he had an AP shell with fuse penetrate his tank, he grabbed the thing (the shell didn't explode) and threw it out of the tank (then it exploded)(apparently he was forced to do this cause the crew would not budge unless he threw out the shell). Another thing I read about the high velocity 57mm T-34 guns - they were so powerful also that the shells went clean through the pz4 and pz3

Anything over 120mm is going to be very slow as 120mm is about the maximum size that a human loader can handle unassisted.

Rounds have gotten lighter since WWII because they are made up of a greater proportion of aluminum sabots by mass.
I'm pretty sure 120mm rounds in WWII weighed around 50 pounds. Not quite easy to lift. I would say in WWII, the maximum size that a loader could handle unassisted would be 90mm.
And you forgot to mention propellant volume, which slowed down the RL superpershing's RoF quite a bit.

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Good post and all but I think most people who can aim are more concerned with getting a -25% RNG roll.

Penetration appears to be on a gaussian curve (penetration models for wargames are on a linear curve, which I think is more realistic) so it's quite rare to get -25%.

Chieftan, I find it interesting that your lack of experience in certain ammunition types reflects your service experience. Your disregard of HESH being the main one.

Where do I disregard it? I point out that it doesn't try to penetrate the armor, which is true. I never said it didn't work or wasn't effective. And for the record, the munition type is still in US Army service today, we just call it "HEP" for some reason.

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Perhaps if you actually had sound knowledge of the era you would have remembered to include shatter gap. Then again the game doesn't include it so why should you.

Basically. I could do a thorough treatise, I guess, on all the factors which affect penetration, from face hardening through blunt-nosed rounds through shatter gap, but it doesn't seem particularly relevant in a 'lite' article like this one.

I like the write-up. Thanks for sharing. In regards to damage after penetration, I think the damage should be much more significant. A single penetration should cause a significant emotional event to the players as well. My suggestion would be to reduce "Hit Points" significantly to about a maximum of about 600 for a Maus and scale everyone else accordingly, as well as killing/injuring crew members and equipment much more easily. It'd keep people from complaining about the drastic differences between tiers.